1. Field of the Invention
The present invention relates to an information processing apparatus comprising a recording-reproducing apparatus which is based on the principle of a scanning tunnel microscope (STM).
2. Related Background Art
In recent years, memory materials form the nucleus of electronics industries employing products such as computers and their related instruments, video discs, digital audio discs, and the like and development of such materials is also under active progress. The performance demanded for memory materials may differ depending on uses, but may generally include:
(1) high density and large recording capacity; PA1 (2) rapid response speed of recording and reproducing; PA1 (3) small power consumption; PA1 (4) high productivity and low cost; and the like.
Heretofore, semiconductor memories or magnetic memories employing semiconductor or magnetic material as the base material have been predominant. However, with the recent advancement of laser techniques, inexpensive and high density recording media with optical memory employing an organic thin film formed from an organic dye, photopolymer, and the like are now being introduced into the field.
On the other hand, a scanning tunnel microscope (hereinafter abbreviated as STM) capable of observing directly the electron structure of the surface atoms of a conductor has recently been developed [G. Binnig et al., Helvetica Physica Acta, 55, 726 (1982)], and it has become possible to measure both single crystalline and amorphous materials with high resolving power for a real space image. Further, it has the advantage that observations can be made at low power without incurring damage by exposing the medium to current.
Further, an STM can be actuated in air and applied to various materials, and therefore a broad scope of applications is expected.
STM is based on the phenomenon that tunnel current flows when a metal probe (probe electrode) and an electroconductive substance approach each other at a distance of about 1 nm with a voltage applied therebetween. This current is very sensitive to the distance change between the metal probe and the electroconductive substance, and by scanning the probe so that the tunnel current is maintained to be constant, the surface structure of the real space can be drawn and, at the same time, a variety of information concerning the electron cloud of the surface atoms can be read. In this case, a resolving power of intrasurface direction is about 1 .ANG..
Accordingly, by applying the principle of STM to recording and reproducing, it is possible to record and reproduce information with a high density on an atomic order (several .ANG.).
Typical methods of recording and reproducing include recording by making the surface state of a recording layer change using either a particle beam (electron beam or ion beam), high energy electromagnetic waves such as x-rays etc. or energy rays as visible light or ultraviolet light, etc. Reproducing is performed using STM. Alternatively, as a recording layer, a thin layer containing a material having a memory effect to a voltage-current switching characteristic, such as an organic compound with a .pi.-electron system or a chalcogenite is used and recording and reproducing are performed using STM. (See Japanese Patent Laid-Open Application No. 63-161552 or No. 63-161553). In this method, the probe electrode which is used in recording and reproducing is required to move precisely above an information-recorded portion of the recording medium. An outline of the position control in this method is as follows.
The size of a recorded bit is on an atomic order (several .ANG.). Therefore, a probe is moved above a desired recording portion of a recording medium in a direction perpendicular to a recording surface of the recording medium at a distance of 10 mm to 1 .mu.m by a piezoelectric element, a stepping motor, an inchworm or by hand (this movement is called coarse movement). The probe is then moved in the atomic order range of 1 .mu.m to 0.1 mm by another driving mechanism, such as a piezoelectric element or the like. (This movement is called fine movement).
The position control of coarse movement and fine movement is based on a standard scale which is provided on a recording medium in advance and contains some information about position.
For example, as a standard scale for fine control, Japanese Patent Laid-Open Applications No. 1-53363 and 1-53364 disclose a method which utilizes atomic period based on a regular arrangement of atoms in the surface of a recording medium, for tracking.
As a piezoelectric element capable of moving a probe and a recording medium, lead titanate zirconate (PZT) is often used because it can enhance the amount of a displacement. PZT, however has a non-linear voltage-displacement characteristic and non-linear hysteresis.
It is possible to perform precise position control of a probe because PZT has a linearity in voltage-displacement characteristic toward fine displacement. However, it is difficult to perform a precise position control for coarse displacement because such linearity is not realized as the amount of displacement increases. Further, it is difficult to obtain a crystal without defects or cracks over the range of more than micron order, when a standard scale for fine adjustment using the above crystalline cell is used.